Duplicated object not showing when rendered
Could you post some screenshots of the screen about the problems? 

However, it may be enough to move the project to a new project file and see if the problem recurs, alternatively you can also make a copy of the object and delete the previous one, first I would tell you to try these simple procedures.
Alternatively, do the objects have applied textures? Does the project have lights? Are they correctly positioned?
Is the camera positioned correctly, does the field of view cover both positioned objects?

Add screenshots of your work screen to make a certain problem more understandable.

In the new project have you tried to import only what you see in the render? to import I mean through the '' append '' section from the blender library.
So to import only what you are interested in a project.
  However, you should also check the maximum view of both the camera and the viewport, or rather being in perspective the image maybe the camera does not get to physically see the most distant object. look at my screenshot